Class-D amplifiers may be categorized into pulse width modulation (PWM) type and sigma-delta (ΣΔ) modulation type, and the ΣΔ-based class-D amplifiers may be further categorized into 1-bit ΣΔ class-D amplifier and 1.5-bit ΣΔ class-D amplifier, where the terms “1-bit” and “1.5-bit” represent the degrees of quantization, or otherwise called “resolution”. More specifically, “1-bit” means that the input of the ΣΔ class-D amplifier is quantized into two logical levels, such as ‘0’ and ‘1’, and “1.5-bit” means that the input of the ΣΔ class-D amplifier is quantized into three logical levels, such as ‘−1’, ‘0’ and ‘1’. FIG. 1 is a block diagram of a conventional 1-bit ΣΔ class-D amplifier 100, which includes a filter 102 to filter an input consisting of signals VINP and VINN opposite in phase to each other to produce signals SOP and SON, a 1-bit quantizer 104 to quantize the signals SOP and SON to produce a digital signal consisting of signals QuP and QuN, and a switching logic 106 to drive a power stage 108 to produce an output Vout for a load according to the signals QuP and QuN produced by the quantizer 104. The output Vout of the ΣΔ class-D amplifier 100 is the difference between two voltages OUTP and OUTN produced by the output stage 108. For better signal-to-noise ratio (SNR), the sigma-delta modulation scheme further includes a feedback mechanism to feed back the voltages OUTP and OUTN in the output stage 108 to the filter 102. The class-D amplifier with 1-bit sigma-delta modulation (ΣΔ-based class-D amplifier) has good SNR compared to that with pulse width modulation, but suffers from instability and high switching loss.
Specifically, a conventional 1-bit ΣΔ modulator is only stable to 50% modulation, which greatly limits the maximum output power to 0.25× of theoretical full-scale. This is because the output Vout from the 1-bit ΣΔ class-D amplifier 100 has only two voltage levels, say +5V and −5V, so that a minimum voltage variation fed back to its input is 10V. This will cause significant increase of the input of the filter 102, leading to abnormal operation of the filter 102 due to saturation thereof. Consequently, the SNR is lowered.
In addition, at least 64× oversampling is need for a conventional 1-bit ΣΔ class-D amplifier to achieve sufficient audio-band SNR, so the typical output data rate is higher than 2 MHz, which increases the switching loss and degrades the power efficiency. For more details on ΣΔ class-D amplifiers, readers are referred to U.S. Pat. Nos. 7,170,340, 6,724,830 and 6,472,933.
Therefore, it is desired a more stable and less switching loss ΣΔ class-D amplifier.